Peroxymonosulfuric acid, which is sometimes called Caro's acid, is represented by the chemical formula of H2SO5 and is conventionally known to have excellent bleaching effect and sterilizing effect. In light of the strong oxidation ability of peroxymonosulfuric acid, expectations are placed on the uses as an etching agent or pickling agent for surface treatment of metals such as copper or copper alloys, a sterilization agent, and an agent for wastewater treatment.
It is well known that peroxymonosulfuric acid can be produced by the reaction of hydrogen peroxide with concentrated sulfuric acid. However, the production of peroxymonosulfuric acid is accompanied by generation of large quantities of heat, and the peroxymonosulfuric acid solution thus produced is too unstable to be stored for a long period of time. Accordingly, the peroxymonosulfuric acid has been employed in the laboratory, but scarcely put to industrial use.
Then, some production methods are proposed to eliminate the shortcoming of poor stability of the peroxymonosulfuric acid solution. To solve the problem of poor stability of the generated peroxymonosulfuric acid, on-site production methods (capable of production near the place where the product is to be put to practical use) are proposed so that the product can be used in the shortest possible time after completion of the production and the loss due to the decomposition can be minimized.
In consideration of the decomposition of peroxymonosulfuric acid by significant heat generation after hydrogen peroxide is mixed with concentrated sulfuric acid, a method is disclosed where the reaction time is restricted to several seconds and the reaction product thus obtained is cooled to normal temperatures or lower and subsequently diluted with cold water. This method needs a lot of energy and a cooling unit, so that the method is not considered to be suitable for the mass production of peroxymonosulfuric acid although the method is applicable to the small-scale production (see PTL 1).
There is also proposed a method where hydrogen peroxide is allowed to react with concentrated sulfuric acid in a vessel equipped with a stirrer, and immediately after completion of the reaction, the reaction product thus obtained is continuously added to cyanogen-containing wastewater. According to this method, hydrogen peroxide and concentrated sulfuric acid are inevitably added to the high-temperature peroxymonosulfuric acid solution that has been just generated with evolution of heat. Consequently, the yield of peroxymonosulfuric acid is drastically decreased by the decomposition of hydrogen peroxide (see PTL 2).
A method of producing peroxymonosulfuric acid in a hermetically sealed tube type reaction chamber is proposed. This method employs the improved small-sized reaction vessel in consideration of the problems in the prior art that the plant for manufacturing the peroxymonosulfuric acid becomes too large, the cooling facilities are too bulky, and the on-site apparatus costs too much. According to this method, the unit for reacting hydrogen peroxide with concentrated sulfuric acid can be made compact, but the problem that the generated peroxymonosulfuric acid is decomposed by heat generated during the reaction still remains unsolved (see PTL 3).
In order to improve the yield of peroxymonosulfuric acid in the reaction, a method is disclosed where hydrogen peroxide is added in some portions to concentrated sulfuric acid. In this method the reaction temperature is controlled using cold water (water from a chiller) while hydrogen peroxide is stepwise added in some portions, which will consequently cause the problem that the apparatus is made larger in size and more complicated. In addition, the above-mentioned method does not take any steps to stabilize the generated peroxymonosulfuric acid (see PTL 4).
To stabilize the generated peroxymonosulfuric acid solution, there is proposed a method of controlling the pH of the solution within the pH range of 0.5 to 2.0. Although the solution may be stabilized by the pH control within the above-mentioned range, cooling operation becomes essential. The pH control of the peroxymonosulfuric acid solution, which is industrially produced in large quantities by the on-site production method, requires the cooling step because a large amount of heat is generated. It is not easy to make the apparatus compact. Further, the pH control requires a large amount of alkali, which is not favorable in economical terms (see PTL 5).
In order to produce peroxymonosulfuric acid, the apparatus employing the method of reacting highly concentrated sulfuric acid with highly concentrated hydrogen peroxide is conventionally known. When the batch production is tried to obtain a large quantity of peroxymonosulfuric acid, the increase in temperature due to heat generation at the time of reaction cannot be easily controlled, so that the yield of peroxymonosulfuric acid is unfavorably decreased in the synthesis. There is also the problem that the stability of the generated peroxymonosulfuric acid is generally so poor that long-term storage thereof will become difficult.
To prevent the above-mentioned problems, a variety of apparatuses for continuously producing peroxymonosulfuric acid adjacent to the place where the product is to be used (so-called “on site”) are proposed.
The PTL 2 discloses a reaction vessel having two reactant transporting pipes symmetrically disposed and a jacket which allows the reaction product to overflow when the amount of product exceeds a certain level. However, this apparatus is not provided with sufficient cooling performance. As a result, when the production scale is industrially increased, the decrease of reaction yield or the increase of cost related to the apparatus will become inevitable.
Further, the PTL 3 discloses the method for producing peroxymonosulfuric acid efficiently by improving the shape of the reaction chamber. However, there is neither specific description about the cooling means against the generation of heat, nor description about the means for preventing the concentrated solution of peroxymonosulfuric acid from coming in direct contact with the operators in the event that the concentrated solution leaks out.